3.2 Enthalpy, Entropy, and Spontaneous Change
- Page ID
- 32255
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We've looked at the two thermodynamic factors factors that determine whether or not a chemical reaction will actually occur - enthalpy (ΔH) and entropy (ΔS).
Remember that in chemical and physical terms, a spontaneous reaction simply means that a reaction will occur - how fast it occurs is not an issue.
Let's summarize our general findings:
| ΔH | positive (endothermic) | reaction tends not to be spontaneous |
| ΔH | negative (exothermic) | reaction tends to be spontaneous |
| ΔS | positive - randomness increases | reaction tends to be spontaneous |
| ΔS | negative - randomness decreases | reaction tends not to be spontaneous |
What if we consider both enthalpy and entropy together?
| ΔH | ΔS | Is the reaction spontaneous? |
| negative | positive | always! |
| positive | negative | never! |
| negative | negative | ??? |
| positive | positive | ??? |
As you can see above, there are two situations where it seems we cannot predict in advance whether a reaction will occur or not - when both ΔH and ΔS are either positive or negative. Is there no way we can predict spontaneity in these cases? Of course there is!